Add insecure option to registry https client
    
    This fixes LP bug #1063793.
    
    This adds a new API config option registry_client_insecure
    (default false) which optionally allows not to specify a ca
    file for registry connections from the API server.
    The default is false so existing behaviour is
    unchanged unless this option is specified.
    Adding this option makes the registry symmetric wrt the API
    with which you can connect using clients with --insecure
    
    Change-Id: I4fa462cd68afaf14f7d7de6c572980a41c0849d6